1. Technical Field
The present disclosure relates to an insulated gate semiconductor device and to a manufacturing method thereof. In particular, the present disclosure regards an insulated gate bipolar transistor (IGBT) with optimized breakdown voltage.
2. Description of the Related Art
IGBTs are electronic devices widely used in various electronic systems. IGBT devices of an advanced type, with low losses and small dimensions, for example of the type with trench-gate terminal or of a planar type (see, for example, FIGS. 1a and 1b), are available on the market.
FIG. 1a shows, in cross-sectional view, a planar IGBT 1 of a known type, which comprises a substrate 2, made of silicon with a doping of a P++ type having a front side 2a and a back side 2b. 
Formed on the front side 2a of the substrate 2 is a buffer layer 4, made of silicon of an N+ type, and formed on top of and in contact with the buffer layer 4 is a drift layer 6, made of silicon with a doping of an N− type. The drift layer 6 houses, at a top face 6a of its own, a body region 8, which extends in depth in the drift layer 6 starting from the top face 6a, and is formed by implantation of dopant species of a P type. Within the body region 8 a source region 10 is formed by implantation of dopant species of an N type, to form a region with N++ doping, which extends in the body region 8 starting from the top face 6a. The planar IGBT 1 further comprises a metal layer 12 formed on the top face 6a of the drift layer 6, in direct contact with the body region 8 and, partially, with the source region 10. The metal layer 12 is moreover separated from portions of the top face 6a that are external, in top plan view, to the body region 8 by means of layers of insulating material 14 and polysilicon 16 set on top of one another, in such a way that the polysilicon 16 is electrically insulated both from the top face 6a and from the metal layer 12 by the layers of insulating material 14. A gate region 18 of the planar IGBT 1 is thus formed. Finally, on the back side 2b of the substrate 2 a collector terminal 19, made of metal material, is formed in direct electrical contact with the substrate 2.
FIG. 1b shows a planar IGBT 20 similar to the planar IGBT 1 of FIG. 1a (elements that are in common are designated by the same reference numbers). The planar IGBT 20 further comprises a barrier layer 22, formed by implantation of dopant species of an N type, which is formed in the drift layer 6 so as to surround the body region 8 externally and be in contact therewith.
In use, the potential that is set up between the barrier layer 22 and the body region 8 acts as a barrier in regard to the holes injected from the region 2 into the drift layer 6. This results in an operative advantage during use of the planar IGBT 20, in so far as the portion of the drift layer 6 close to the body region 8 can be modulated in a more effective way as compared to the planar IGBT 1 of FIG. 1a. The barrier layer 22 reduces the depletion area underneath the body region 8 and generates an accumulation of holes such as to decrease the resistivity of the respective portion of drift layer 6, and hence enables reduction of the saturation voltage VCEsat when the planar IGBT 20 is forward biased.
According to this embodiment, however, since the concentration of spatial charge underneath the barrier layer 22 is high, in conditions of reverse biasing the breakdown voltage decreases. Said reduction in the breakdown voltage can be compensated by modulating the thickness and resistivity of the drift layer 6. This solution, albeit guaranteeing an adequate breakdown voltage, does not guarantee optimal values of the “energy off” (energy consumption at turning-off of the device) and of the peak electrical field underneath the body region 8. A very high electrical field could cause breakdown of the device during the reliability tests. In other words, when the device is in extreme working conditions (high humidity and/or high temperature in conditions of reverse biasing), very high localized electrical fields (i.e., ones well above the critical electrical field of silicon) can cause undesirable and unexpected breakdown.